Scintillators are well known in the radioactive detection arts. A scintillator is a material that converts energy into light. Energy is deposited into the scintillator by penetrating radiation. This energy is then converted into ultra-violet or visible light, which can then be detected with the use of a photo detector such as a photo multiplier tube. Generally, incident penetrating radiation includes high-energy particles and ionizing radiation such as x-rays, gamma rays, alpha particles, beta particles, thermal neutrons, etc.
Scintillators are therefore materials that emit flashes or pulses of light when ionizing radiation such as gamma rays interact with them. Plastic scintillators formed from an aromatic polymer such as polystyrene or polyvinyltoluene are particularly well suited for radiation detection applications. These materials are readily melt-processible and capable of being extruded into a variety shapes and sizes to meet the spatial requirements of the detector involved.
A typical plastic scintillator is composed of a polymer matrix doped with two or more fluorescent organic compounds, a primary dopant and a secondary dopant. Solutions of a polymer and an efficient fluorescent dopant can give a system with close-to-unity quantum yield, a high-efficiency scintillator. The term quantum yield is defined as the probability that an excited state in the polymer leads to the emission of a photon by a dopant, which is generally an organic compound. In Forster's theory of nonradiative energy transfer between molecules in solution, the energy transfer is described by a dipole-dipole interaction in which nonradiative energy transfer occurs between the first excited π-singlet state of the solvent and the solute (i.e., dopant) in the ground state. The strength of this interaction is a function of the natural fluorescence lifetime of the solvent and the mean separation between the solvent and the solute molecule.
Fluorescence can be defined as the emission of a photon by a molecular transition from the molecule's first excited singlet state to its ground state. At high dopant concentrations (e.g. approximately 1% by weight), this process can dominate the emission or quenching process of the solvent. If the dopant's radiative quantum yield is close to unity, the number of photons emitted per solvent-molecule π-electron excitation can approach one even though the radiative quantum yield of the solvent is small (e.g., in the case of polystyrene, for example, 0.07).
Dopants that couple to the primary excitation of the solvent are called primary dopants. The primary dopant serves to raise the photon yield of the solvent plus dopant(s) combination (the number of photons emitted per unit energy deposited in the solvent) and to shift the mean wavelength of the final fluorescence of the scintillator to longer wavelengths. The addition of secondary dopants, in low concentrations (e.g., approximately 0.01% to 0.2% by weight), to the binary system of solvent and primary dopant results in the shifting of the fluorescence wavelength further to the red portion of the spectrum. Secondary dopants do not increase the intrinsic photon yield of the scintillator. By shifting the fluorescence emission to a longer wavelength, however, more photons escape from the scintillator since self-absorption by the scintillator is reduced. The term “technical quantum yield” is often applied to scintillators of finite size. In this case, effects of self-absorption are included in the determination of quantum or detected photon yield of the scintillator.
The most commonly used polymer bases in plastic scintillator are polystyrene and polyvinyltoluene. Monomeric styrene is a liquid, which polymerizes on heating to form the solid plastic polystyrene. When doping of the polystyrene or some alternate polymer matrix, several methods are available including:    1. dissolving the dopant in molten polystyrene;    2. polymerizing a styrene-dopant solution at a low temperature (e.g., 50° C.) with a benzoyl peroxide catalyst;    3. polymerizing a styrene-dopant solution at a medium temperature (e.g., 125° C.-140° C.) over several days without a catalyst;    4. polymerizing a styrene-dopant solution at a high temperature (e.g., 200° C.) without a catalyst for a period approximately 12-15 hours.
The utilization of method 2 above may impair transparency and reduce the efficiency of the scintillator. The size of the plastic scintillator frequently dictates the production method used. For small samples, a monomer is generally subjected to numerous vacuum distillation processes to remove inhibitors. The monomer is then transferred to a vial containing the dopant. The vial is connected to a vacuum system, and the dissolved gas is removed by repeated freeze-pump-thaw cycles. The evacuated vial is sealed, shaken to insure the complete dissolution of the dopant in the monomer, and then placed in an oil bath at 125° C. for several days.
For larger specimens, a generally employed technique is to bubble nitrogen through the monomer to expel the oxygen and then mix in the dopant. The solution is then heated and the polymerization is carried out in a nitrogen atmosphere. For high temperature polymerization, the vacuum-distilled monomer and the dopant are placed in a reaction flask fitted with a reflux condenser and flushed with nitrogen for approximately thirty minutes after which the container is evacuated and sealed. The container is then heated in a bath to over 200 degree. C. for a period of from eight to ten hours.
Typically, the technique for the preparation of plastic scintillators is:    1. purification of the monomer by vacuum distillation;    2. addition of dopants to monomer;    3. removal of dissolved gasses, notably oxygen;    4. complete polymerization in an inert atmosphere or under vacuum; and    5. careful annealing.
A variety of methods are known in the art for producing plastic scintillators. In one method, for example, combining polystyrene pellets, which have been purged with an inert gas, with oil, and then mixing in the dopants can produce a plastic scintillator material. Mineral oil has been utilized with some success. Significantly better results, however, have been achieved when silicone oil was utilized as a plasticising agent to coat the polymer with dopant during mixing. The preferred silicone oil is an aromatic-substituted silicone.
The pellet-oil-dopant combination is compounded or processed in an inert gas atmosphere, argon or nitrogen, to generate plastic scintillating material that can then be pelletized. These scintillator pellets can subsequently be extruded, injection molded and/or exposed to other plastic molding processes to form a scintillator piece of a particular shape or form. This process can also directly extrude scintillating fibers, sheet, or film from the melt, thus avoiding the pelletizing of the new scintillating plastic. By purging the initial polymer pellet stock with argon, the water and oxygen are driven out of the pellets. This and the subsequent use of argon or nitrogen in the processing of the mixture eliminate or reduce the need for a vacuum.
In another method for producing plastic scintillator material, the need for pre-mixing a polymer-pellet oil dopant combination can be eliminated by utilizing an inline coloring and compounding extrusion process. In this method the polymer pellets and all additives (dopants, silicone oil if desired, etc.) can be metered into the processing device and again a plastic scintillator material can be produced. The use of an inert gas purge can also be utilized in association with this method. Scintillator profiles, sheet, film, fiber can be directly produced by this method or the scintillator material can be pelletized for subsequent plastic forming operations.
The present inventors have developed a method for producing a plastic scintillator, which was disclosed in U.S. Pat. No. 5,968,425, “Methods for the Continuous Production of Plastic Scintillator Materials,” which was issued to Bross et al on Oct. 19, 1999. U.S. Pat. No. 5,968,425 is directed toward methods for producing plastic scintillating material, including the use of silicone oil as a plasticising agent to uniformly and homogeneously coat the polymer with the dopants and then processing the mixture to mix the components at the molecular level.
The invention disclosed in U.S. Pat. No. 5,968,425 also includes the implementation of an inert gas blanket (e.g., argon) with respect to the initial polymer and the combination of the polymer and the dopant during processing of the mixture to produce a plastic scintillating material. In the alternative, plastic scintillating material can also be produced by metering a dopant combination, a plurality of polymer pellets together with a regulated stream of inert gas, preferably argon, into a processing apparatus, usually a compounder. The polymer-dopant mixture disclosed in U.S. Pat. No. 5,968,425 can then be compounded in an inert gas atmosphere to produce a plastic scintillating material.
Although adequate for particular uses, the prior art discussed above, including the invention disclosed in U.S. Pat. No. 5,968,425, is limited in applicability and production efficiency. The prior art discussed above, for example, is not optimized for neutron detection. Thus, the present inventors believe that a need exists for an improved scintillator, including fabrication methods thereof, which is optimized for neutron detection. The present inventors have thus developed an improved scintillator, including improved fabrication methods thereof.